1. Field of the Invention
This invention relates to an image forming apparatus and a method for forming an image using the same, and more particularly to an image forming apparatus that enables stable image transfer with enhanced transferability by detecting an electric current (hereinafter simply referred to as current) derived in a transfer unit based on a summation of inflow and outflow of current to and from a photoreceptor as a whole, and a method for forming an image using such image forming apparatus.
2. Description of the Background Art
Generally, there have been known various electrophotography type image forming apparatuses such as copying machines, facsimile machines and printers provided with transfer units as an essential part in which toner is electrically attracted to an electrostatic latent image formed on the surface of a photoreceptor 1 to develop latent image into a toner image and the toner image is electrically attracted to a copy sheet to transfer the toner image thereto. One of such conventional image forming apparatuses is schematically shown in FIG. 3.
As shown in FIG. 3, the image forming apparatus A3 comprises a charging unit 2 for uniformly charging the surface of a photoreceptor (hereinafter merely referred to as a "main charger"), a developing unit 3 for developing an electrostatic latent image, formed on the photoreceptor surface through projection of light that has been modulated based on image data, into a toner image by electrically attracting toner to the latent image, a power supply 5 for supplying electric charges, i.e., applying a certain voltage to a transfer roller 4 in a transfer unit C provided at an appropriate position over the outer circumference of the photoreceptor 1 to electrically attract the toner image to a copy sheet transported to the transfer unit C to transfer the toner image thereto, and a cleaning unit 6 for removing toner residues from the surface of the photoreceptor 1 after the image transfer by the transfer unit C.
More specifically, the main charger 2 includes a U-shaped casing member (shielding member) 2a, a wire electrode 2b made of a metallic material such as tungsten, a power supply (or battery) 2c for applying a voltage of several K-volts to the electrode 2b, and bias voltage means 2d for applying a bias voltage to the casing member 2a.
The photoreceptor 1 is grounded, and when the power supply 2c applies a certain high voltage to the electrode 2b, corona discharge generates between the wire electrode 2b and the photoreceptor 1 to uniformly charge the photoreceptor surface at a positive potential level. It is to be noted that the corona discharge generates over the outer surface of the photoreceptor 1, which is in the form of a drum, since the bias voltage means 2d applies a certain bias voltage to the shielding member 2a.
When the uniformly positively charged surface of the photoreceptor 1 is exposed to projection of light that has been modulated based on image data, the electric resistance on the photoreceptor surface at the exposed area varies according to the intensity level of modulated light, thereby forming an electrostatic latent image on the exposed area on the photoreceptor surface.
Meanwhile, negatively charged toner is electrically attracted to the electrostatic latent image by a developing roller 3b of the developing unit 3 drives by power supply 3a to develop the latent image into a toner image.
Also, electric charges of the power supply 5 are supplied to a copy sheet that has been transported to the transfer unit C by the transfer roller 4 to transfer the toner image onto the statically charged copy sheet (to effect image transfer).
After the image transfer by the transfer unit C, the cleaning unit 6 removes toner residues on the surface of the photoreceptor mechanically and with the use of electrical attraction force of the power 6a.
There has been proposed an idea of incorporating a controller in the above image forming apparatus A3, as disclosed in Japanese Unexamined Patent Publication No. SHO 61-14670. In this publication, the controller indicated by the symbol M in FIG. 3 for controlling a current established in the transfer unit C (hereinafter merely referred to as "transfer current") resulting from inflow and outflow of current, is provided to prevent deterioration of transferability due to change of humidity in the atmospheric air.
It is a well known phenomenon that when the humidity in the atmospheric air rises, moisture absorption of copy sheet is raised, which in turn, lowers its electric resistance. As a result, electric charges supplied to the transfer roller 4 through the copy sheet of low electric resistance during an image transfer and escape to peripheral members around the transfer roller 4, such as guide means for guiding the copy sheet. Thus, rise of humidity lowers image transferability and hence obstructs realization of desirable density of transferred image.
Considering the above phenomenon, the image forming apparatus provided with the controller M is effective when used with copy sheets of low electric resistance (i.e., in a state of high moisture absorption).
Referring back to FIG. 3, the controller M is arranged at a specified position in a ground wire 7 connecting the photoreceptor 1 to the ground to control the transfer current in a predetermined range, and comprises photoreceptor outflow current detector means 8 for detecting a current flowing from the photoreceptor 1 to the ground wire 7 and transfer current control means 19 for controlling output of the power 5 based on a measurement result by the detector means 8.
The controller M causes the outflow current detector means 8 to monitor the current flowing from the photoreceptor 1 and feed-backs to the power supply 5 so as to set the difference between currents in the presence and in the absence of copy sheet in the transfer unit C, as a control value (target value) for controlling the output of the power 5.
However, it is to be noted that the photoreceptor 1 is subject to inflows and outflows of current--besides the transfer current--through other electric charge send/receive means (hereinafter referred to as charge SND/RCV means) such as the developing unit 3 and the cleaning unit 6. Accordingly, in the arrangement of the apparatus A3 in which the change rate or value of transfer current is estimated by merely monitoring the change of photoreceptor outflow current from the photoreceptor 1, the accurate transfer current cannot be detected, resulting in hindrance against desirable control of output of the power supply 5. One of such drawbacks is described with reference to the graph in FIG. 4.
FIG. 4 is a graph showing how the current flowing into the photoreceptor 1 from charge send/receive means (such as the developing unit 3 and the cleaning unit 6) varies according to the state (density-white or black) of an original document image when the surface of the photoreceptor 1 is positively charged and negatively charged toner is attracted to a latent image on the photoreceptor surface.
Specifically, in the case where the original document has no image, i.e., is all white, the negatively charged toner is hardly attracted to the photoreceptor 1, resulting in no or insignificant change of current due to electrical attraction of toner. Since the main charger 2 is positively charged and the photoreceptor 1 is grounded, a current I.sub.mc shown in FIGS. 3 & 4 flows to the photoreceptor 1 from the main charger 2.
On the other hand, if the original document image is all black (high image density), a great amount of negatively charged toner moves toward the positively charged photoreceptor surface from the developing unit 3, thus remarkably reducing the current I.sub.dev (indicated by the arrow .rarw. in FIG. 3) flowing into the photoreceptor I.sub.dev from the developing unit 3. Also, the current flowing from the photoreceptor 1 to the cleaning unit 6 increases since the potential of photoreceptor 1 becomes high in the case of transferring image of high density, which, in turn, remarkably decreases a resultant current I.sub.cl (indicated by the arrow .fwdarw. in FIG. 3) that flows from the cleaning unit 6 to the photoreceptor 1.
In this way, despite the fact that the transfer current I.sub.t increases as a whole due to decrease of currents such as I.sub.cl and I.sub.dev when the original document image of high density is to be transferred, the conventional image forming apparatus A3 attempts to control the transfer current I.sub.t simply based on the photoreceptor outflow current I.sub.pc flowing from the photoreceptor 1 through the ground wire 7. However, such control is not feasible as FIG. 4 clearly illustrates the situation in which the image density of the original document changes. Accordingly, an attempt to control the transfer current I.sub.t based on the photoreceptor outflow current I.sub.pc alone results in partial incapability or deterioration of image transfer and a hindrance against a stable image transfer.